» Note to Firefox users:
When you click a link to an XPI file, Firefox thinks you are trying to install an extension to Firefox and brings up the install dialog. This is not going to work. Instead, Right-click on the links for the extension and save to disk, then install from within Thunderbird's Tools->Add-ons window.

» Note to Safari users:
When you click on the http link, Safari may try to load the XPI file as plain text. To avoid this, right-click (or control-click) on it and select Download Linked File from the contextual menu.